1. Field of the Invention
The present invention relates to an ultrasonic sensor that is used, for example, as a back-up sensor for automobiles.
2. Description of the Related Art
An ultrasonic sensor is a sensor that uses ultrasonic waves to perform sensing. The ultrasonic sensor includes a piezoelectric transducer, which intermittently sends ultrasonic waves (transmission waves) and receives reflected waves (reception waves) from an object. The distance between the ultrasonic sensor and the object is measured via the transmission/reception wave signals. The piezoelectric transducer is disposed on the inner bottom surface of a bottom portion of a tubular container. The tubular container is mounted in a supporting member such that the outer bottom surface, which serves as a vibrating surface, of the bottom portion is located substantially opposite to the object.
Known ultrasonic sensors include a single piezoelectric device acting as both a transmitter and a receiver. In this case, as an ultrasonic sensor and an object approach each other, reverberations of one of a transmission signal and a reception signal affect the other signal more strongly and cause a problem in that the object cannot be properly detected. In a configuration proposed to solve this problem, two piezoelectric devices, one that acts as a transmitter and the other that acts as a receiver, are disposed in respective tubular containers. However, while the effects of reverberating vibrations are reduced, this configuration has led to the problems of an increased number of components and increased assembly costs.
Japanese Unexamined Patent Application Publication No. 10-206529 discloses an ultrasonic transmitting-receiving apparatus in which two piezoelectric devices are disposed in respective tubular containers that are connected together to form an integral housing. This apparatus is configured such that reverberations of both transmission and reception signals are prevented from affecting each other.
FIG. 10 is a cross-sectional view showing an ultrasonic sensor of the ultrasonic transmitting-receiving apparatus disclosed in Japanese Unexamined Patent Application Publication No. 10-206529. In an ultrasonic sensor 90, two tubular containers 92a and 92b and a connecting portion 92c that connects the two tubular containers 92a and 92b form an integral housing 92 having conductivity. An end of the ultrasonic sensor 90 is closed by bottom portions 92a1 and 92b1 acting as vibrating surfaces, while openings at the other end of the ultrasonic sensor 90 are covered with circuit boards 92f and 92g. The two tubular containers 92a and 92b are connected to each other, through the connecting portion 92c, at side portions near the openings of the tubular containers 92a and 92b. Piezoelectric devices 92d and 92e are disposed on the respective inner bottom surfaces of the bottom portions 92a1 and 92b1 of the housing 92. Shielded wires W1 and W2 are connected to the circuit boards 92f and 92g, respectively. Live lead wires 92h1 and 92i1 extending from the circuit boards 92f and 92g, respectively, are connected to the piezoelectric devices 92d and 92e, respectively. Ground lead wires 92h2 and 92i2 extending from the circuit boards 92f and 92g, respectively, are connected to portions of the bottom portions 92a1 and 92b1, respectively, the portions being adjacent to the piezoelectric devices 92d and 92e. 
The housing configuration of the ultrasonic sensor disclosed in Japanese Unexamined Patent Application Publication No. 10-206529 enables the tubular containers to be connected together, through the connecting portion 92c, at the side portions near the openings where there are fewer vibrations. Therefore, reverberations caused by one of the piezoelectric devices do not substantially affect the other piezoelectric device. A possible reason for which there are fewer vibrations in the side portions near the openings is that the openings, which are covered with the circuit boards 92f and 92g, are subjected to restraining forces of the circuit boards 92f and 92g. 
It is preferable that general ultrasonic sensors have a housing configuration that minimizes interference with vibrations created by piezoelectric transducers. However, as in Japanese Unexamined Patent Application Publication No. 10-206529, covering the openings of the tubular containers with the circuit boards causes interference with such vibrations. If an ultrasonic sensor has a configuration in which openings are not covered, side portions of tubular containers vibrate most strongly at portions near the openings, in response to the vibrations of piezoelectric transducers. Therefore, if tubular containers are connected to each other at side portions near the openings, it is more likely that reverberations of vibrations created by one piezoelectric device affect the other piezoelectric device.